MAX1565 Datasheet, PDF (14/26 Pages) Maxim Integrated Products – Small, High-Efficiency, Five-Channel Digital Still Camera Power Supply

Small, High-Efficiency, Five-Channel
Digital Still Camera Power Supply
Detailed Description
The MAX1565 is a complete digital still camera power-
conversion IC. It can accept input from a variety of
sources including single-cell Li+ batteries, 2-cell alkaline
or NiMH batteries, as well as systems designed to accept
both battery types. The MAX1565 includes five DC-to-DC
converter channels to generate all required voltages:
1) Synchronous rectified step-up DC-to-DC con-
verter with on-chip MOSFETs—This typically sup-
plies 3.3V for main system power.
2) Synchronous rectified step-down DC-to-DC con-
verter with on-chip MOSFETs—Powering the step-
down from the step-up output provides efficient (up
to 90%) buck-boost functionality that supplies a reg-
ulated output when the battery voltage is above or
below the output voltage. The step-down can also
be powered from the battery.
3) Auxiliary DC-to-DC Controller 1—Typically used
for 5V output for motor, strobe, or other functions as
required.
4) Auxiliary DC-to-DC Controller 2—Typically sup-
plies LCD bias voltages with either a multi-output
flyback transformer, or boost converter with charge-
pump inverter. Alternately may power white LEDs
for LCD backlighting.
5) Auxiliary DC-to-DC Controller 3—Typically sup-
plies CCD bias voltages with either a multi-output
flyback transformer, or boost converter with charge-
pump inverter.
The MAX1565 can also operate with MAX1801 slave DC-
to-DC controllers if additional DC-to-DC converter chan-
nels are required. All MAX1565 DC-to-DC converter
channels employ fixed-frequency PWM operation.
In addition to multiple DC-to-DC channels, the
MAX1565 also includes overload protection, soft-start
circuitry, adjustable PWM operating frequency, and a
power-OK (POK) output to signal when the step-down
converter output voltage (for CPU core) is in regulation.
Step-Up DC-to-DC Converter
The step-up DC-to-DC converter channel generates a
2.7V to 5.5V output voltage range from a 0.9V to 5.5V
battery input voltage. An internal switch and synchro-
nous rectifier allow conversion efficiencies as high as
95% while reducing both circuit size and the number of
external components. Under moderate to heavy loading,
the converter operates in a low-noise PWM mode with
constant frequency. Switching harmonics generated by
fixed-frequency operation are consistent and easily
filtered.
The step-up is a current-mode PWM. An error signal (at
COMPSU) represents the difference between the feed-
back voltage and the reference. The error signal pro-
grams the inductor current to regulate the output voltage.
At light loads (under 75mA when boosting from 2V to
3.3V), efficiency is enhanced by an idle mode in which
switching occurs only as needed to service the load. In
this mode, the inductor current peak is limited to typically
200mA for each pulse.
Step-Down DC-to-DC Converter
The step-down DC-to-DC converter channel is opti-
mized for generating output voltages down to 1.25V.
Lower output voltages can be set by adding an addi-
tional resistor (see the Applications Information sec-
tion). An internal switch and synchronous rectifier allow
conversion efficiencies as high as 95% while reducing
both circuit size and the number of external compo-
nents. Under moderate to heavy loading, the converter
operates in a low-noise PWM mode with constant fre-
quency. Switching harmonics generated by fixed-fre-
quency operation are consistent and easily filtered.
The step-down is a current-mode PWM. An error signal
(at COMPSD) represents the difference between the
feedback voltage and the reference. The error signal pro-
grams the inductor current to regulate the output voltage.
At light loads (under 120mA), efficiency is enhanced by
an idle mode in which switching occurs only as needed
to service the load. In this mode, the inductor current
peak is limited to 150mA (typ) for each pulse.
The step-down remains inactive until the step-up DC-
to-DC is in regulation. This means that the step-down
DC-to-DC on/off pin (ONSD) is overridden by ONSU.
The soft-start sequence for the step-down begins 1024
OSC cycles after the step-up output is in regulation. If
the step-up, step-down, or any of the auxiliary con-
trollers remains faulted for 200ms, all channels turn off.
The step-down also features an open-drain SDOK out-
put that goes low when the output is in regulation.
Buck-Boost Operation
The step-down input can be powered from the output of
the step-up. By cascading these two channels, the step-
down output can maintain regulation even as the battery
voltage falls below the step-down output voltage. This is
especially useful when trying to generate 3.3V from 1-cell
Li+ inputs, or 2.5V from 2-cell alkaline or NiMH inputs, or
when designing a power supply that must operate from
both Li+ and alkaline/NiMH inputs. Compound efficien-
cies of up to 90% can be achieved when the step-up and
step-down are operated in series.
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